Dynamic range enhancement and phase-ambiguity elimination in wavelength-interrogated interferometric sensor

We present a method to resolve the phase-ambiguity problem in interferometer sensors. The proposed method is based on operating a wavelength-interrogated two-wave interferometric sensor close to the critical sensitivity point. We show that in this vicinity, changes in the transmission spectrum take place at two sensitivity scales. One of them is highly sensitive and periodic while the other scale has low sensitivity but is monotonic and quasi-linear and serves for recording the phase periods. The dynamic range of the sensor can thus be increased by 1-2 orders of magnitude as compared to ordinary unbalanced interferometric sensors. The method thus avoids the need for the continuous tracking of data while sensing. Critical sensitivity can exist in a hetero-modal interferometer where the propagation takes place within paths for which the dispersion properties are different. Calculations and detailed numerical simulations are presented for the design of a porous-silicon (PSi) waveguide interferometer which exhibits critical sensitivity. The waveguides of the interferometer are of rib-type and are composed of a guiding layer of low porosity on a cladding layer with higher porosity.